A micro-machined hydrophone employing a piezoelectric body combined on the gate of a field-effect transistor
Introduction
Hydrophones are underwater acoustic receivers that serve as a key component of sonar systems. The applications of hydrophones are usually determined by the limits of the sensitivity, usable frequency range, and size. Recently, emphasis has been placed on the miniaturization of hydrophones [2], [3], [4] as the large size of conventional hydrophones restricts the physical design of hydrophone arrays that are used in most sonar systems [5], [7].
A significant reduction of the physical dimensions of hydrophones with good receiver characteristics at low frequencies (from several Hz to several tens of kHz) would facilitate a variety of compact arrays such as vector sensor arrays [6] and towed array sonar systems [7], as well as the applications for improved autonomous underwater vehicles (AUVs), unmanned underwater vehicles (UUVs) [6], [8], and sonobuoys [9]. Furthermore, small hydrophones are less prone to the effects of acoustic diffraction, which is advantageous for precise measurements [5], [10].
However, miniaturization of most conventional piezoelectric hydrophones is limited by the loss of sensitivity and high impedance at low frequencies. The sensitivity reduces with the dimensions of the piezoelectric body (hereafter referred to as the piezo-body) [11], because the measured signal in response to the acoustic pressure is proportional to the voltage at the electrode, which is in turn proportional to the integrated charge over the surface of the piezo-body. High impedance at low frequencies of the piezo-body leads to a low-frequency roll-off [12] and undesirable loss of signal due to the effect of parasitic impedances of the extended signal line and interconnections [11], [12], [17].
Monolithic integration of a field-effect transistor (FET) in close proximity to piezoelectric materials has advantages for signal conditioning and processing due to the early conversion of the signal impedance into the low output impedance of the FET [13], [14], [15], [16], [17], [18], [19]. Hydrophones that exploit monolithic integration of the FET near to the piezoelectric polymer (typically either polyvinylidene fluoride (PVDF) or poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE)), referred to as piezoelectric oxide semiconductor FETs (POSFETs), have been reported for ultrasonic applications [13], [14], [15], [16], [17]. With POSFETs, the partially induced charge on the extended FET gate (i.e., the parasitic capacitance between the extended FET gate and substrate) results in a loss of sensitivity. Fabrication techniques to reduce the parasitic capacitance have been developed [15], [16], [17], and recently, a modified POSFET with P(VDF-TrFE) patterned directly on the FET gate has been demonstrated for touch-sensing applications [18], [19]; however, the sensitivity was not sufficient for acoustic-sensing applications.
Changes in the bound surface charge density, and corresponding variation in the electric field from the piezo-body, do not depend on the size of the piezo-body. In this paper, we describe a micro-machined hydrophone in which the piezo-body is combined directly on the gate of the FET (termed a piezoelectric gate on a field-effect transistor, PiGoFET). With the PiGoFET transduction mechanism, the sensitivity is decoupled from the dimensions of the piezo-body, because the channel current of the FET is directly modulated by the electric field from the bound surface charge of the piezo-body. An acoustic receiving thin plate on the piezo-body of the PiGoFET is used as mechanical leverage to amplify the input acoustic pressure, which allows the PiGoFET hydrophone to achieve a high sensitivity using a small thin plate and a smaller piezo-body.
The feasibility of the PiGoFET mechanism for hydrophone applications has been demonstrated using a macroscale device as a proof-of-concept [20]. In this work, a micro-PiGoFET hydrophone is realized as a direct exploitation of the PiGoFET mechanism for hydrophone miniaturization. Based on the dimensionless characteristics of the PiGoFET, a miniaturized hydrophone device was designed with micron-scale dimensions that can be fabricated using a conventional micro-electro-mechanical system (MEMS) and complementary metal-oxide semiconductor (CMOS) fabrication processes.
Hybrid bonding integration was used to combine the piezoelectric MEMS structure (i.e., piezo-body on a silicon thin plate) with the CMOS device (i.e., FET) to fabricate the micro-PiGoFET. Most piezoelectric materials in MEMS technology involve high temperatures for deposition and crystallization, as well as elements that are not compatible with standard CMOS fabrication [21]. The piezoelectric MEMS and CMOS processes used in this work were carried out with separate wafers, which were then combined using eutectic wafer bonding at a low temperature without requiring electrical activation [22]. This bonding approach completes the micro-PiGoFET using reliable process technologies, while maintaining a high level of CMOS-compatibility. These developments thus provide scope to fabricate high-performance miniaturized hydrophone systems with additional monolithically integrated CMOS electronics (i.e., system-on-chip functionality), which can be cost-effective via batch processing.
Section snippets
Operating principle
Fig. 1 shows a schematic diagram of the micro-PiGoFET hydrophone. The micro-PiGoFET consists of a FET with a piezo-body combined on the gate of the FET, which is compressed under a pre-stressed acoustic receiving thin plate. The thin plate mechanically amplifies the acoustic input pressure onto the piezo-body to provide high sensitivity, and also reduces the acoustic impedance mismatch between the water and the piezo-body, since it can be considerably more flexible than the thickness-mode
Piezo-body on silicon thin plate
Fig. 6(a) shows the piezoelectric MEMS fabrication process used to form the acoustic receiving thin plate with a microscale PZT body at the center. A wet thermal oxidation was used to form a 2.4-μm-thick oxide layer on a 4-inch n -type silicon-on-insulator (SOI) wafer. A Pt/Ti layer was deposited using sputtering, followed by a 2.5-μm-thick PZT (Pb(Zr0.52,Ti0.48)O3), which was deposited using the sol-gel method [30]. A Pt layer was deposited on the PZT using e-beam evaporation to form a
Underwater receiver characteristics
Fig. 8 shows the underwater experimental setup to measure the receiver characteristics of the micro-PiGoFET hydrophone at low frequencies. A circular, acrylic waveguide [35] with a low-frequency transmitter (Benthowave, BII-7534) was used to calibrate the micro-PiGoFET hydrophone using the planar standing waves. Low-frequency cutoff occurred because of the properties of the transmitter, and the high-frequency cutoff of the waveguide occurred due to the radial mode [35], which restricted
Conclusion
We have demonstrated a micro-machined hydrophone using a piezoelectric gate on a field-effect transistor (PiGoFET). A microscale piezo-body was combined directly on the gate of a FET, with mechanical pressure amplification via a receiving plate so that the sensitivity could be decoupled from the dimensions of the sensor. A theoretical analysis of the dimensionless characteristics of the PiGoFET was described, which showed that significant improvements in the sensitivity could be achieved using
Acknowledgments
The authors thank Prof. Chungwol Kim of Andong University for his advice in developing the measurement circuit for the experiments. This research was supported primarily by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP; No. 2011-0030075), and partially by the Agency for Defense Development (ADD) grant funded by the Defense Acquisition Program Administration (DAPA) of Korea (No. UD130017DD).
Min Sung received his B.Sc. in Mechanical Engineering from Pohang University of Science and Technology (POSTECH) in 2007, and is currently enrolled in the integrated Ph.D. program in Mechanical Engineering at POSTECH. His main research activities concern the modeling and design of acoustic transducers, focusing on MEMS piezoelectric sensors and FET-based sensors integrated with piezoelectricity. His research interests include the design and fabrication of MEMS acoustic devices based on PZT
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Min Sung received his B.Sc. in Mechanical Engineering from Pohang University of Science and Technology (POSTECH) in 2007, and is currently enrolled in the integrated Ph.D. program in Mechanical Engineering at POSTECH. His main research activities concern the modeling and design of acoustic transducers, focusing on MEMS piezoelectric sensors and FET-based sensors integrated with piezoelectricity. His research interests include the design and fabrication of MEMS acoustic devices based on PZT thick film.
Kumjae Shin received his B.Sc. in Mechanical Engineering from Hanyang University in 2007. He is currently enrolled in the integrated Ph.D. program in Mechanical Engineering at POSTECH. His research activities include the design and fabrication of FET-based transducers for scanning probe microscopy (SPM) and acoustic sensor applications, focusing on MEMS microphones. His research interests involve the design and fabrication of electrets and FETs for MEMS acoustic/vibration devices.
Wonkyu Moon received his B.Sc. in Mechanical Engineering from Seoul National University in 1984 and a M.Sc. from the Korea Advanced Institute of Science and Technology in 1986. He received a Ph.D. from the University of Texas, Austin, in 1995. In 1996, he joined the Samsung Advanced Institute of Technology, where he researched micro-actuators and the application of SPM technology to data storage devices. Since joining POSTECH in 1998, his research has been on electromechanical transducers for acoustic and nano-positioning applications, the development of new electromechanical transducers and analysis tools for improving their design including advanced transduction materials.